1. Field of the Invention
The present invention relates to a network apparatus which performs data communication by wireless, and a program for the network apparatus.
2. Description of the Related Art
In recent years, wireless LAN (Local Area Network) systems have been coming into wide use.
In such a wireless LAN system, a limited number of wireless channels are shared among a plurality of nodes. Accordingly, with increase of nodes, communication conditions become severer. In particular, firstly, it can be imagined that the case where no free wireless channel can be detected in carrier sense occurs more frequently. Secondly, it can be imaged that the case where crosstalk is caused by concurrent transmission occurs more frequently.
Description will be made in more detail. Assume that wireless communication is initiated between nodes. Before transmitting data, a transmission-side node receives radio waves in an intended channel, and checks whether another node is sending a radio wave or not. This process is referred to as “carrier sense”. As a result of the carrier sense, it is concluded that the wireless channel is free when there is no radio wave from any other node. Then, the transmission-side node starts transmitting data to be transmitted. On the contrary, when another node is sending a radio wave, it is concluded that the wireless channel is in use. Thus, the transmission-side node waits till the channel allows data transmission. When the number of nodes using one and the same wireless channel increases, the probability that the wireless channel is in use becomes higher. As a result, the probability that each node waits till the wireless channel allows data transmission also becomes higher. Thus, there occurs a problem that the throughput becomes lower.
In addition, when the number of nodes using one and the same wireless channel increases, the probability that two or more nodes substantially concurrently carry out carrier sense as described above becomes higher. In this event, all the nodes conclude that the wireless channel is free, and start transmitting data substantially concurrently. As a result, crosstalk occurs. The occurrence of crosstalk leads directly to a problem that a data error is generated. Further, when such a data error is detected, there is a problem that the throughput becomes lower due to a retransmitting process carried out by a mechanism mounted for performing the retransmitting process for recovery from an error.
As a method for relaxing the problem, a technique for changing the receiving sensitivity dynamically has been adopted in the background art. That is, the receiving sensitivity is increased when only a weak radio wave arrives, while the receiving sensitivity is reduced when a powerful radio wave arrives. Thus, according to this technique, only a powerful radio wave is received selectively under an environment where the powerful radio wave and a weak radio wave arrive simultaneously.
When a node performs carrier sense with this technique, a weak radio wave may be radiated from a node which is not a target of communication. Even in such a case, when a radio wave from a node which is a target of communication is powerful, the receiving sensitivity of the node performing the carrier sense becomes so low that only the radio wave from the node which is a target of communication is received selectively. Accordingly, there is no fear that it is concluded that the wireless channel is in use due to the weak radio wave from the node which is not a target of communication. Thus, the probability that the node waits till it is allowed to perform data transmission becomes low correspondingly, so that reduction in throughput becomes difficult to occur.
Two or more nodes may perform such carrier sense substantially concurrently, and start to transmit data concurrently. Even in such a case, the receiving sensitivity of each node becomes low when the radio wave from a node which is a target of communication is powerful and the radio wave from a node which is not a target of communication is weak. As a result, each node performing the carrier sense is allowed to selectively receive only the radio wave from the node which is a target of communication. Thus, there is no fear that crosstalk occurs, so that a data error caused by the crosstalk or reduction in throughput caused by the process for retransmitting the data becomes difficult to occur.
That is, when the technique for changing the receiving sensitivity dynamically as described above is adopted, a powerful radio wave can be received by priority so that the adverse effect of the existence of a weak radio wave can be excluded.
When the receiving sensitivity is changed dynamically as described above, a more powerful radio wave is received by priority. Therefore, when it is intended only to improve the communication condition between a node in question and another node, a radio wave between those nodes had better be more powerful. However, if a powerful radio wave is radiated immoderately, it may interfere with communication between other nodes. On the contrary, when it is intended only to prevent interfere with communication between other nodes, a radio wave between a node in question and another node had better be made weaker. However, there is a problem that there is apt to occur a failure such as intermittence of communication between the node in question and another node.
To solve such a problem, for example, JP-A-H7-087093 discloses a technique in which information about the transmitter-side transmitting power is transmitted from the transmitter to a receiver, while the transmitter-side transmitting output level requested by the receiver is fed back from the receiver to the transmitter, so that the transmitter and the receiver can perform transmitting and receiving in optimal conditions without setting the transmitter-side transmitting output level to be too high or too low.
JP-A-H7-087093 is referred to as a related art.
However, when the technique disclosed in JP-A-H7-087093 is adopted, both the transmitter and the receiver have to support a dedicated power control protocol. Even when one of them supports the power control protocol, the transmitting power cannot be optimized if the other does not support the power control protocol.
Some data exchanged between nodes have a comparatively high degree of importance, but some data have a comparatively low degree of importance. In the background art, these data are transmitted on one and the same transmitting output level without any special distinction. Some data exchanged between nodes are data comparatively easy to retransmit (data that can be transmitted any number of times or data having a small data volume), but some data are data comparatively difficult to retransmit (data that can be transmitted only once or data having a large data volume). In the background art, these data are transmitted on one and the same transmitting output level without any special distinction.
For this reason, there is a problem as follows. That is, when a neighbor node transmits data having a comparatively low degree of importance or data comparatively easy to retransmit in a situation that data having a comparatively high degree of importance or data comparatively difficult to retransmit are to be transmitted/received between nodes, the data having a comparatively low degree of importance or the data comparatively easy to retransmit interfere with the transmission/reception of the data having a comparatively high degree of importance or the data comparatively difficult to retransmit. As a result, a data error or reduction in throughput occurs. In this case, the data error may cause no severe problem on the data having a comparatively low degree of importance or the data comparatively easy to retransmit. Those data are easy to retransmit if necessary. However, the data error may cause a severe problem on the data having a comparatively high degree of importance or the data comparatively difficult to retransmit. There may be no measure of retransmission about those data.